How to add a realistic flare
You can use the Fog Glow in the compositor to produce a nice glare for the bright sun as follows :



I've used two levels of the glare node which isn't ideal (as it's not the most efficient method) but it provided a simple means of filtering out the 'bright' elements of your original image (since it's a JPG is doesn't include brightness values > 1.0 - in your actual scene you could use an emission material with, say, emission strength of 20 or so - this way the threshold on a single Glare node can do the job!) to be multiplied up by the 'Divide' node (by dividing by a small value - eg, divide by 0.05 to multiply it by 20).

The final Glare node then adds the actual glare on those elements picked out by that first stage to produce something like the following :



To include this in the render pipeline my preference would be to enable the Emit pass in the Render Layer. This provides you with an additional output from the Render Layers node for the emission materials only. You then only need to use a single Glare node for the emission before adding it into the rest of your render.



You can vary the glow by adjusting the Divide node and/or the Glare node or by varying the brightness of any emissive material in the scene. If you want to exclude certain emissive materials from the 'glare' then you can use the Glare node's Threshold to filter those out - ie, setting it to, say, 5 will result in any emission at a strength of less than 5 not being subject to glare while any greater than 5 being subject to the glare.

An alternative is to apply the glow directly to the rendered Image and to rely on the emission strength to provide the glow. This makes for a simpler compositing set up but you lose the ability to easily adjust the glow effect - you would need to adjust the actual emission strength rather than being able to simply adjust the "multiplier" (Divide node in the previous example).